Dose and time effects of estrogen on expression of neuron-specific protein and cyclic AMP response element-binding protein and brain region volume in the medial amygdala of ovariectomized rats.

Although estrogen has been shown to be neuroprotective, studies concerning its effect on some behaviors are contradictory, reporting both ameliorative and detrimental effects. A factor involved in hormone efficacy is the estrogen regimen. We reported an effect of 10 microg estrogen for 14 days on the cyclic AMP response element-binding protein (CREB) pathway, including brain-derived neurotrophic factor, in rat medial amygdala (MeA). To determine the effects of estrogen on neuronal numbers and brain region volume in MeA and central nucleus of the amygdala (CeA), we used stereology to test the effect of various estrogen regimens on the number of neuron-specific protein (NeuN)-labeled neurons and brain region volume of MeA and CeA. Ovariectomized rats were injected with vehicle for 14 days, 2.5 microg estradiol benzoate (E2) for 4 or 14 days, or 10 microg estrogen for 14 days. Because NeuN-labeled neuronal number may be related to neuronal survival and upregulation of CREB signaling, we tested the effect of these regimens on levels of phosphorylated CREB (pCREB) labeling in the MeA and CeA. The 2.5 microg estrogen for 14 days regimen increased the mean number of NeuN-labeled neurons and pCREB-labeled cells in the MeA compared to vehicle or 2.5 microg for 4 days. There was an increase in volume of the MeA with 2.5 microg estrogen for 14 days compared to vehicle or 2.5 microg for 4 days. No differences in these parameters were seen in CeA. These data indicate a neuroanatomical heterogeneity of a time effect of estrogen on cells expressing NeuN and pCREB in the MeA versus CeA.

Knockout of p75NTR does not alter the viability of striatal neurons following a metabolic or excitotoxic injury.

Following metabolic or excitotoxic injury to the striatum, there is de novo expression of the low-affinity p75 neurotrophin receptor (p75NTR). The novel expression of this pan neurotrophin receptor in rodents occurs within the lesion core and surrounding area, creating a division between viable and nonviable tissue. The present series of experiments sought to elucidate whether the p75NTR expression seen following metabolic and excitotoxic injury alters neuronal viability within the striatum. Toward this end, we compared the extent of striatal lesion created with quinolinic acid (QA) or 3-nitropropionic acid (3-NP) in p75NTR null and wild-type mice. Using stereological techniques, we found that the lesion volume and neuronal cell counts between p75NTR null and wild-type mice were similar 1, 2, and 4 weeks post-QA or -3-NP lesion. The results indicate that the expression of p75NTR within reactive astrocytes in the mouse striatum is not a key factor in protecting neuronal cell death following metabolic and excitotoxic insults.

GFAP knockout mice have increased levels of GDNF that protect striatal neurons from metabolic and excitotoxic insults.

In response to injury and degeneration, astrocytes hypertrophy, extend processes, and increase production of glial fibrillary acidic protein (GFAP), an intermediate filament protein located within their cytoplasm. The present study tested the hypothesis that GFAP expression alters the vulnerability of neurons to excitotoxic and metabolic insult induced by 3-nitroproprionic acid (3-NP), an irreversible inhibitor of mitochondrial complex II activity or the excitotoxin quinolinic acid (QA). In this respect, adult GFAP knockout mice (KO) and wild-type control mice (WT) received unilateral intrastriatal injections of 3-NP (200 nmol/microl) or QA (100 nmol/microl) and were killed 1, 2, or 4 weeks later. Lesion volume and neuronal counts were quantified using unbiased stereologic principles. For both QA and 3-NP lesions, a significant decrease in lesion volume and an increase in striatal projection neurons were seen in GFAP KO mice compared with WT mice. Enzyme-linked immunoassay analysis revealed increased basal levels of glial cell derived neurotrophic factor (GDNF) relative to WT mice. In contrast, no differences were observed in the expression of ciliary neurotrophic factor or nerve growth factor. These data strongly suggest that the expression of GFAP is implicated with the production of GDNF to a degree that confers neuroprotection after an excitotoxic or metabolic insult.

Excitotoxic and metabolic damage to the rodent striatum: role of the P75 neurotrophin receptor and glial progenitors.

After injury, the striatum displays several morphologic responses that may play a role in both regenerative and degenerative events. One such response is the de novo expression of the low-affinity p75 neurotrophin receptor (p75(NTR)), a gene that plays critical roles in central nervous system (CNS) cell death pathways. The present series of experiments sought to elucidate the cellular origins of this p75(NTR) response, to define the conditions under which p75(NTR) is expressed after striatal injury, and how this receptor expression is associated with neuronal plasticity. After chemical lesions, by using either the excitotoxin quinolinic acid (QA) or the complex II mitochondria inhibitor 3-nitropropionic acid (3-NP), we compared the expression of the p75(NTR) receptor within the rat striatum at different survival times. Intrastriatal administration of QA between 7 days and 21 days postlesion induced p75(NTR) expression in astrocytes that was preferentially distributed throughout the lesion core. P75(NTR) immunoreactivity within astrocytes was seen at high (100-220 nmol) but not low (50 nmol) QA doses. Seven and 21 days after 3-NP lesions, p75(NTR) expression was present in astrocytes at all doses tested (100-1,000 nmol). However, in contrast to QA, these cells were located primarily around the periphery of the lesion and not within the lesion core. At the light microscopic level p75(NTR) immunoreactive elements resembled vasculature: but did not colocalize with the pan endothelium cell marker RecA-1. In contrast, p75(NTR)-containing astrocytes colocalized with nestin, vimentin, and 5-bromo-2-deoxyuridine, indicating that these cells are newly born astrocytes. Additionally, striatal cholinergic neurons were distributed around the lesion core expressed p75(NTR) 3-5 days after lesion in both QA and 3-NP lesions. These cells did not coexpress the pro-apoptotic degradation enzyme caspase-3. Taken together, these data indicate that striatal lesions created by means of excitotoxic or metabolic mechanisms trigger the expression of p75(NTR) in structures related to progenitor cells. The expression of the p75(NTR) receptor after these chemical lesions support the concept that this receptor plays a role in the initiation of endogenous cellular events associated with CNS injury.